The loop is executed exactly four times and the line in question executed once per trip around the loop with n decremented by 2 each time.

The line "if n > 0 then" is arrived at with n=2, 0, -2, -4 and so "algor(z,n)" is executed once with n = 2, and in executing this with n=2 the line in question is executed twice, and the line "algor(z,n)" is never reached.